I feel like you're using the word viable wrong. If you're talking dictionary definition, he's viable at all levels of play (50% win rate in Korean plat+ and made an appearance in LCK play this year). He has clear and well-defined weaknesses, which kinda makes him tough in a meta where most of the top tier and 2ND tier junglers are guys specifically strong at amumu'so weakness(graves, Xin, camille, lee, nidalee), but it's also a meta where the strongest guys in
every
role is a squishy damage guy, so being able to wrap the enemy squishes while your squishies use all their moves is really strong. And his scaling with liandriy's is pretty dope and real does put a hurting on people, but it definitely comes at the cost of like...hoping graves and the mid laner don't ruin your whole game. But yeah. There's a reason he has the highest win rate of all junglers gold and below and it'd because he can take advantage of being the only guy with cc lol.
Top 5 play rate gold and below in top lane:
Garen
Darius
Jax
Riven
Gangplank (teemo and nasus are close)
Mid lane
Zed
Ahri
Yasuo
Akali
Fizz (LeBlanc over akali in gold and plat)
Bot lane
Jhin
Miss fortune
Ezreal
Jinx
Kaia
Support
Thresh (=()
Morgana
Pyke
Nami
Lux
Jungle
Lee
Graves
Yi
Kayn
Jax
So uh...in short, ban graves, play amamu, hope your support is Morgana and win every game until you decide hitting plat is a stupid waste of time.
Sometimes a sandwich is just a sandwich.